1. Field of the Invention
The present invention relates to embedded magnetic components, and in particular, to embedded magnetic components with improved isolation performance.
2. Description of the Related Art
Power supply devices, such as transformers and converters, involve magnetic components such as transformer windings and often magnetic cores. The magnetic components typically contribute the most to the weight and size of the device, making miniaturization and cost reduction difficult.
In addressing this problem, it is known to provide low profile transformers and inductors in which the magnetic components are embedded in a cavity in a resin substrate, and the necessary input and output electrical connections for the transformer or inductor are formed on the substrate surface. A printed circuit board (PCB) for a power supply device can then be formed by adding layers of solder resist and copper plating to the top and/or bottom surfaces of the substrate. The necessary electronic components for the device may then be surface mounted on the PCB. This allows a significantly more compact and thinner device to be built.
In US2011/0108317, for example, a packaged structure including a magnetic component that can be integrated into a printed circuit board, and a method for producing the packaged structure, are described. In a first method, illustrated in FIGS. 1A to 1E, an insulating substrate 101, made of epoxy based glass fiber, has a cavity 102 (FIG. 1A). An elongate toroidal magnetic core 103 is inserted into the cavity 102 (FIG. 1B), and the cavity is filled with an epoxy gel 104 (FIG. 1C) so that the magnetic component 103 is fully covered. The epoxy gel 104 is then cured, forming a solid substrate 105 including an embedded magnetic core 103.
Through-holes 106 for forming primary and secondary side transformer windings are then drilled in the solid substrate 105 on the inside and outside circumferences of the toroidal magnetic component 103 (FIG. 1D). The through-holes are then plated with copper, to form vias 107, and metallic traces 108 are formed on the top and bottom surfaces of the solid substrate 105 to connect respective vias together into a winding configuration (FIG. 1E) and to form input and output terminals 109. In this way, a coil conductor is created around the magnetic component. The coil conductor shown in FIG. 1E is for an embedded transformer and has left and right coils forming primary and secondary side windings. Embedded inductors can be formed in the same way, but may vary in terms of the input and output connections, the spacing of the vias, and the type of magnetic core used.
A solder resist layer can then be added to the top and bottom surfaces of the substrate covering the metallic surface terminal lines, allowing further electronic components to be mounted on the solder resist layer. In the case of power supply converter devices, for example, one or more as transistor switching devices and associated control electronics, such as Integrated Circuit (ICs) and Operational Amplifiers (Op Amps) may be mounted on the surface resist layer.
Devices manufactured in this way have a number of associated problems. In particular, air bubbles may form in the epoxy gel as it is solidifying. During reflow soldering of the electronic components on the surface of the substrate, these air bubbles can expand and cause failure in the device.
US2011/0108317 also describes a second technique in which epoxy gel is not used to fill the cavity. This second technique will be described with respect to FIGS. 2A to 2F.
As illustrated in FIG. 2A, through-holes 202 are first drilled into a solid resin substrate 201 at locations corresponding to the interior and exterior circumference of an elongate toroidal magnetic core. The through-holes 202 are then plated up to form the vertical conductive vias 203 of the transformer windings, and metallic caps 204 are formed on the top and the bottom of the conductive vias 203 as shown in FIG. 2B. A toroidal cavity 205 for the magnetic core is then routed in the solid resin substrate 201 between the conductive vias 203 (FIG. 2C), and an elongate toroidal magnetic core 206 is placed in the cavity 205 (FIG. 2D). The cavity 205 is slightly larger than the magnetic core 206, and an air gap may therefore exist around the magnetic core 206.
Once the magnetic core 206 has been inserted into the cavity 205, an upper epoxy dielectric layer 207 (such as an adhesive bondply layer) is added to the top of the structure, to cover the cavity 205 and the magnetic core 206. A corresponding layer 207 is also added to the bottom of the structure (FIG. 2E) on the base of the substrate 201. Further through-holes are drilled through the upper and lower epoxy layers 207 to the caps 204 of the conductive vias 203, and plated, and metallic traces 208 are subsequently formed on the top and bottom surfaces of the device as before (FIG. 2F).
As noted above, where the embedded magnetic components of FIGS. 1A-1E and 2A-2F are transformers, a first set of windings 110, 210 provided on one side of the toroidal magnetic core form the primary transformer coil, and a second set of windings 112, 212 on the opposite side of the magnetic core form the secondary windings. Transformers of this kind can be used in power supply devices, such as isolated DC-DC converters, in which isolation between the primary and secondary side windings is required. In the example devices illustrated in FIGS. 1A-1E and 2A-2F, the isolation is a measure of the minimum spacing between the primary and secondary windings.
In the case of FIGS. 1A-1E and 2A-2F above, the spacing between the primary and secondary side windings must be large to achieve a high isolation value, because the isolation is only limited by the dielectric strength of the air, in this case in the cavity or at the top and bottom surfaces of the device. The isolation value may also be adversely affected by contamination of the cavity or the surface with dirt.
For many products, safety agency approval is required to certify the isolation characteristics. If the required isolation distance through air is large, there will be a negative impact on product size. For mains reinforced voltages (250 Vms), for example, a spacing of approximately 5 mm is required across a PCB from the primary windings to the secondary windings in order to meet the insulation requirements of EN/UL60950.
The inventors of the invention described and claimed in the present application discovered that it would be desirable to provide an embedded magnetic component device with improved isolation characteristics, and to provide a method for manufacturing such a device.